The invention pertains to the image quality produced by a printhead, and in particular to a diagnostic method for anticipating the onset of unacceptable image quality.
Inkjet printers are efficient, quiet and can produce high quality print images in a relatively inexpensive manner. Such quality is achieved by sweeping a printhead with a large number of inkjet nozzles over a print medium and ejecting ink droplets onto the medium in a matrix array of minute ink dot patterns. As is well known in the art, a printed image is made up of discrete picture elements or pixels. The closer together the pixels, the more readily can it be perceived by a human eye as a continuous image. The quality of the print image is then determined by assuring that each ink droplet has a precise volume of ink that is applied to a specific location on the print medium without smearing.
Color printing requires a plurality of printheads, each delivering ink of a single color, to be referred to as a primary color. To produce an image on a medium such as paper, the primary colors are delivered to a pixel location as determined by the color requirements at that area. There are methods well known in the art to provide that that the primary colors are present on the medium as discrete dots that do not smear or mix, but nevertheless register with a human eye as mixed colors contributing to high quality full color images. Unless a desired color happens to coincide with a primary color, an image color therefore relies for its formation on more than one printhead. In a typical image, there are typically 600 pixels per inch or 24 pixels per mm.
While inkjet printers produce high quality photo-like images, it is well known to those skilled in the art that dot size and placement accuracy degrade with time due to mechanical and electrical fatigue factors associated with the printhead nozzles of such printers. Such nozzle fatigue factors can include a buildup of firing residue on resistors, and droplet directionality can change because orifice plates may be scratched through continued wiping by service station cleaning apparatus. They are generally manifested in pattern-like printing errors that become quite noticeable.
Some prior solutions err on the side of replacing the printhead sooner than necessary, which is wasteful. These include dictating a specific lifetime for a printhead based on its time in service, or on the volume of ink ejected. Attempts have been made to predict image quality based on nozzle health or algorithms based on nozzle performance. One such method relies on amplifying defects, that is, detecting them sooner than they would occur in normal use. This can be done by using fewer passes than normal of the printhead over the medium. However, a printhead can be useful for some time after failing a diagnostic test of this kind.
Other solutions are backward looking, noting defects only after they have occurred in prints. However, once defects have become obvious to customers, the loss of customer goodwill may result.
There is therefore a need for a just-in-time solution which can detect the onset of defects as late as possible in the lifetime of a printhead while precluding the possibility of defective images when the printhead is in regular use.
The invention is a test image printed predominantly by an ink jet printhead under evaluation. However, another printhead, assumed to be functioning normally, also contributes to the test image. The image has a plurality of tiles or regions of a selected uniform solid color with a selected brightness and a selected hue. The tile is selected to be most sensitive to the types of defect encountered with regular images. It has been found by experiment that certain configurations of test images are most sensitive to defects. In particular, an ink dot density selected so that adjacent ink dots just overlap provides an optimum sensitivity. Defects are observed in such test images serve to indicate the imminence of similar defects in regular images. Test images are run according to a schedule determined by an algorithm. The algorithm is based on various inputs, including the age of the printhead, the volume of ink printed by the printhead, the time elapsed since an immediately preceding diagnostic print, and a physical evaluation of the printhead. Previously, such an algorithm would be used as a direct indicator of when a printhead should be replaced. However, producing a defect-free test image allows the useful life of the printhead to be extended.